Eggshells are more than just a handy source of protection for their developing inmates. James Stewart and his colleagues from East Tennessee State University, USA, and the Station Biologique de Paimpont, France, explain that eggshell is also an essential source of calcium for common lizard embryos. ‘81% of the calcium contained in hatchlings is mobilized from eggshell,’ says the team. However, some northern populations of common lizards have opted for a less conventional form of reptile reproduction: they carry their eggs throughout gestation and give birth to live young. However, they have also done away with the egg's calcium outer layer, leaving the embryos without their additional calcium supply. Stewart and his colleagues explain that these live-born young must somehow extract calcium from their surroundings through a placenta structure to supply their additional requirements, but it wasn't clear how. So, the team decided to find out whether both types of embryo used the same structures to extract calcium from their environment (p. 2999).
According to the team, egg-incubated lizards extract calcium from their eggshells via the chorioallantoic membrane – the vascular structure that coats the egg's internal surface and supplies oxygen to egg-incubated and live-born young – and the splanchnopleure – which connects the yolk to the embryo's gut – through two proteins: calbindin-D28K and plasma membrane calcium ATPase (PMCA). They decided to look for evidence of these proteins in both structures in the shelled and shell-less embryos.
Having collected egg-laying common lizards near Lourdes in the south of France and live-bearing common lizards near Rennes in northern France, the team found that both live-born and egg-incubated young absorbed calcium through calcium transporting proteins in the chorioallantoic membrane and the splanchnopleure. However, the shell-less young extract calcium from the uterine secretions surrounding them, while the embryos developing inside their calcium carbonate shells absorb calcium from the shell.
Stewart and his colleagues explain that live birth has evolved several times in common lizards, and the lizards have benefited from the versatility of the chorioallantoic membrane, which is equally capable of extracting calcium from eggshells and secretions surrounding them in the mother's uterus. ‘The mechanism of embryonic calcium acquisition functions independently of the immediate source of calcium,’ says the team.
